The removal of undesired traces of acid or acid forming substances from the air is of central importance in many industrial applications as well as in applications in personal and object protection.
Acids or acid forming substances develop corrosive or other destructive effects on the surface of objects which could impair or permanently destroy the entire object.
An example of such an acid forming substance is sulphur dioxide which is generated by burning fossil fuels. The generated sulphur dioxide is subsequently released into the atmosphere where it mixes with fresh air and is then reintroduced into buildings in a diluted form as part of the fresh air supply.
On surfaces or in combination with moisture films, the sulphur dioxide transforms to sulphurous acid, later sulphuric acid. In combination with ammonia as an alkaline substance, salts are produced, which for example in the form of ammonium sulphate crystals on the surface of optical components, such as illumination masks in the semiconductor production, can result in significant interference during the production process.
Further examples of acids are hydrogen chloride (HCl) and hydrogen fluoride (HF) which attack both metallic and non-metallic surfaces. For example hydrogen fluoride HF attacks the glass fibre structures which are commonly used in cleanroom filter systems for particle filtration. Hydrogen fluoride partly reacts with the boric components of the glass fibres and creates boron tri-fluoride, which is a volatile component which can escape into the air and can result in undesired doping effects in silicon based semiconductor structures during manufacturing processes.
An example of damaged objects are, to name a few, metallic or semiconductor structures and substrate surfaces in semiconductor manufacturing and processing as well as the surfaces of objects with a mineral constitution such as marble or enamel as objects of common use or art.
A further area in which acids or acid forming substances can occur is the breathing air or breathing protection with respect to the inhalation of acidic, caustic gases, for example during a fire or disaster. A typical example is the release of hydrogen chloride HCl during the burning of chloride containing plastics such as polyvinylchloride (PVC).
A number of processes have been described for removing acids and acid forming substances from airstreams.
Most processes, according to the state of the art, rely on sorption materials that are based on unmodified or chemically modified carbon where the acidic gases are stored intermediately on the surface, and by means of chemical transformation, stable, non-volatile products should be produced.
A disadvantage of sorption on activated carbon is always that, in order to achieve reliable and efficient operation, a well controlled ratio of the surface deposit to the total pore volume at ideal temperature and humidity conditions must be carried out. In addition it is unavoidable in such activated carbon systems that, due to the pore structure, an undesired co-adsorption of organic, hydrophobicizing substances occurs which can not only limit the adsorption capability for acids but can completely prevent it which occurs frequently which deprives from a forecast and control of performance.
Another disadvantage of the sorption process on activated carbon is that this is a multi-step process comprising transport to the active carbon core, migration into the pores, intermediate storage and chemical reaction. Even though this process is thermodynamically supported by the enthalpy of the reaction, some steps along the chain can only occur very slowly due to the transport kinetics which consequently results in a relatively poor sorption performance.
It was also found that undesired and uncontrollable reactions can occur on the surface of the activated carbon which, by means of a surface reaction, changes a harmless or mildly active substance into a higher oxidized, corrosive substance. One example is the partial conversion of nitrogen dioxide NO2 on the surface of activated carbon into nitrous acid HNO2.
Patent EP 0 991 470 B1 discloses a filter for clean air for the cleaning of air streams of gaseous acidic substances, such as sulphur dioxide or ammonia, which comprises a highly air permeable, three dimensional carrier on which ion-exchange spheres are attached whereby the carrier is a large pored, reticulated polyolefin foam and the attachment of the ion-exchange spheres is achieved via heating. The ion-exchange spheres can either be strong alkaline anion exchange resins or strong acidic cation exchange resins.
Patent WO 01/70391 discloses a filter material with adsorbing characteristics comprising a carrier layer and a first layer of adsorbing material which is connected with the carrier layer. In addition, the filter material comprises a second or a second and a third adsorbing layer whereby all individual adsorbing layers form one total adsorbing layer. One or two of the adsorbing layers consist preferably of impregnated activated carbon material. Additionally, one or two of the adsorbing layers consist preferably of ion-exchange materials. The implementation of ion-exchange resins results in a very advantageous extension of the life times with high efficiency of the filter materials.
The disadvantages of the technologies described in this document result from the previously mentioned disadvantages of the activated carbon components within the material. Furthermore, it is known that slightly alkaline anion exchange resins exhibit poor performance in binding acid forming substances such as sulphur dioxide (SO2). Strong alkaline OH-containing anion exchange resins show chemical and thermal instability since they continuously produce volatile amino compounds resulting from a continuous degradation process of the polymer structure.
The above described disadvantages bring about that the described materials are not suitable for the required wide-banded removal of acids and acid forming substances. A further disadvantage is the release of amino compounds which are very strong odorous substances which makes the implementation of the materials impossible in ventilation systems in residential buildings or production sites.
The release therefore prevents use of the described materials in the area of breathing protection as well as the area of semiconductor manufacturing where the release of compounds could result in process disruptions.